Nonthermal Plasma Processes for Sustainable Synthesis of Metallic Titanium Nanoparticles

Titanium and its alloys are crucial resources in a wide range of applications including aerospace materials, medical implants, catalysts, nanomanufacturing, electronics, and optics.¹ Conventional industrial production of titanium powders involves extraction and refining procedures requiring long processing time and high temperature, which introduces environmental and economic concerns.² Due to titanium's propensity to strongly oxidize at the nanoscale, most of the existing reports utilize additional coating to protect the metallic core, which induces large particle size variation and complicates the process.³ To synthesize titanium nanoparticles with high purity and narrow size distribution with environmentally benign and simplified methods is highly desired.<br/>Plasma techniques are paving the way for the green and rapid fabrication of nanomaterials in the renewable energy sector. Here, we demonstrated the vapor deposition of metallic titanium nanoparticles from titanium tetrachloride (TiCl₄) in a one-step capacitively coupled nonthermal plasma reactor with a rapid process timescale of 20 ms, which has not been reported up to date. The fabrication process was evaluated and optimized based on the particle properties (crystallinity and chemical purity), production rate, and conversion efficiency from TiCl₄ to Ti. We investigated the plasma parameters and chamber conditions (pressure and gas flows) on the dissociation and chemical reaction of the precursors. The as-synthesized particles exhibit tunable average sizes around 20-30 nm controlled by the in-flight timescale determined by the chamber pressure. Hydrogen addition was revealed in our study as a crucial condition to capture the chlorine and facilitate the formation of metallic nanoparticles. Ar was used as the carrier gas and the effect of the gas flow ratios (Ar/TiCl₄ and Ar/H₂) on the production rate was explored. Optimal gas flow ratios were identified under which the process yield was maximized. The nanoparticles show polycrystalline features as suggested by the lattice fringes and SAED patterns determined by electron microscopy. The plasma power input plays a key role in facilitating the crystallization of the particles, with improved crystal features under higher power. The chemical purity (element composition and oxidation states) was elucidated by X-Ray Photoelectron Spectroscopy (XPS). Metallic Ti 2p peaks dominate in the XPS spectrum in an air-free environment with negligible shifts to oxides (Ti³⁺ or Ti⁴⁺), which indicates the high purity of the nanoparticles fabricated using this process. We systematically discussed the role of nonthermal plasma in transforming and reacting chemical precursors into metallic titanium nanoparticles and how the process variables influence material production and properties.<br/>This work expands the scope of nonthermal plasmas for metallic nanoparticle fabrication, providing a green and simplified route for the rapid processing and formation of monodispersed nanomaterials. The discussion provides insights into the application of nonthermal plasmas to achieve high-efficiency and low-cost material production as well as precise manipulation of material properties. The development and optimization of plasma techniques may offer more benefits in terms of energy sustainability and efficiency.<br/>This work was supported by the Minnesota Futures Grant Program by the Office of the Vice President for Research (OVPR).<br/><br/>References<br/>1. Kulkarni, M. <i>et al.</i> Titanium Nanostructures for Biomedical Applications. <i>Nanotechnology</i> <b>26</b>, (2015).<br/>2. Seagle, S. R. Titanium processing. <i>Encycl. Br.</i> 1–8 (2018).<br/>3. Zhang, D. <i>et al.</i> Carbon-Encapsulated Metal/Metal Carbide/Metal Oxide Core-Shell Nanostructures Generated by Laser Ablation of Metals in Organic Solvents. <i>ACS Appl. Nano Mater.</i> <b>2</b>, 28–39 (2019).